The ecoregions shown here have been derived from Omernik (1987) and from refinements of Omernik's framework that have been made for other projects. These ongoing or recently completed projects, conducted in collaboration with the U.S. EPA regional offices, other federal agencies, state resource management agencies, and groups from neighboring North American countries, involve refining and subdividing ecoregions. Designed to serve as a spatial framework for the research assessment, and monitoring of ecosystems and ecosystem components, ecoregions denote areas within which ecosystems (and the type, quality, and quantity of environmental resources) are generally similar. By recognizing the spatial differences in the capacities and potentials of ecosystems, ecoregions stratify the environment by its probable response to disturbance (Bryce et al. 1999). These general purpose regions are critical for structuring and implementing ecosystem management strategies across federal agencies, state agencies, and nongovernmental organizations that are responsible for different types of resources within the same geographical areas (Omernik et al. 2000, McMahon et al. 2001).

The approach used to compile these maps is based on the premise that ecological regions can be identified through the analysis of the patterns and the composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Wiken 1986; Omernik 1987, 1995). These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another regardless of the hierarchical level. Because of possible confusion with other meanings of terms for different levels of ecological regions, a Roman numeral classification scheme has been adopted for this effort. Level I is the coarsest level, dividing North America into 15 ecological regions, whereas at Level II the continent is subdivided into 50 classes (CEC 1997, 2006). Level III, a subdivision of Level II, has been even further subdivided into Level IV regions for the conterminous United States. More detailed explanations of the methods used to define the USEPA ecoregions are given in Omernik and Griffith 2014, Omernik 1995, 2004, and Omernik et al. 2000. The applications of the ecoregions are explained in Bryce et al. 1999 and in reports and publications from the state and regional projects (e.g., Griffith et al. 2007, Griffith et al. 1994, and Omernik et al. 2000).

Griffith, G.E., J.M. Omernik, T.F. Wilton, and S.M. Pierson. 1994. Ecoregions and subregions of Iowa: A framework for water quality assessment and management. The Journal of the Iowa Academy of Science 101(1):5-13.